Two kinds of PDF password
The open password (also called user password) is the real lock: without it the PDF cannot be opened, and its pages, text and images are stored encrypted inside the file. Anyone who receives the file needs the password to read it.
The permissions password (also called owner password) only matters when you limit printing, copying or editing. Readers that respect these limits show the document but block those actions until the permissions password is entered. Most PDF apps respect them, but not every program does, so treat the limits as a polite request and the open password as the actual protection.
AES-256 or AES-128
AES-256 is the encryption of the current PDF standard and the default here. It opens in Adobe Acrobat and Reader, the PDF viewers in Chrome, Edge, Firefox and Safari, on iPhone and Android, and in current office software. Choose AES-128 only if the file must open in a very old PDF reader. With AES-128 the password can only use Latin letters, numbers and common symbols.
A strong encryption only helps with a strong password. Use at least 10 characters, mix words and numbers, and do not reuse a password from your email or bank. Send the password through a different channel than the file, for example by phone or text message.
Your file stays with you
The encryption runs entirely in your browser. Neither the PDF nor the password is sent anywhere, and the password is not saved in the page link or on this device. If you forget it, nobody can recover it for you, so keep a copy of the original file or store the password in a password manager.